In the process of drilling oil and gas wells, drilling mud is injected into the center of the drill string to flow down to the drillbit and back up to the surface in the annulus between the outside of the wellbore and drillstring to carry the drill cuttings away from the bottom of the wellbore and out of the hole. The drilling mud is also used to prevent blowouts or kicks when the borehole is kept substantially full of drilling mud by maintaining head pressure on the formations being penetrated by the drillbit. A blowout or kick occurs when high pressure fluids such as oil and gas in downhole formations are released into the wellbore and rise rapidly to the surface. At the surface these fluids can potentially release considerable energy that is hazardous to people and equipment. The drilling muds used for drilling oil and gas wells have been developed with weighting (densifying) agents to provide sufficient head pressure to prevent the initial release of high pressure fluids and gases from the formation. However, density alone does not solve the problem as the drilling mud may drain into one or more formations downhole lowering the volume of drilling mud in the hole and, thus, head pressure for the wellbore. The situation where drilling mud is draining into one or more formations is called “lost circulation.”
Lost circulation and stuck pipe are two of the most costly problems faced while drilling oil and gas wells. To reduce the likelihood of lost circulation, particles of “lost circulation material” (commonly called “LCM”) are added to drilling muds to plug the formations into which the drilling mud is being lost. It is a simple and elegant solution in that the particles flow toward the leaking formation carried by the drilling mud and then collect in the leaking formation at the side of the wellbore. Eventually, however, when losses of drilling fluid become excessive, it is necessary to stop drilling and install a string of casing to seal off the portion of the existing wellbore so that drilling may re-commence at the bottom of the casing string. Installing casing or liner creates substantial costs as drilling is suspended while the casing is installed and cemented. Expenses for the installing casing string are only part of the cost as the day rates for the drilling rig and personnel continue while further progress on drilling stops.
It should also be noted that the interior dimension of the hole is reduced as each successive string of casing is added to the borehole. It is common to require a minimum diameter within the casing at the target zone in order to produce hydrocarbons that may be present when considering the space needed for tubing, valves, pumps and other equipment. Thus, the borehole is initially drilled substantially oversized anticipating successively smaller wellbore dimensions with each string of casing. It is also incumbent on the drilling crew to reach milestones before a new string of casing is installed so as to preserve final interior dimension of the casing.
The second area of substantial added cost for well drilling is when pipe gets stuck in the hole. This includes stuck drillstrings and stuck casing and stuck wireline logging tools. These pipes are often stuck because permeable zones allow the differential pressure of the drilling fluid hydrostatic pressure and formation pressure to stick the drill string against the filter cake with greater force than can be applied to pull the pipe loose. In addition, wellbore collapse and debris from the spalling or breakout of rock often cause stuck pipe.
Casing drilling is an operation where the drill string is actual casing pipe instead of the normal smaller diameter drill pipe. This casing drilling process has been partially effective at reducing lost circulation and improving wellbore stability through what has been called the smear effect. The smear effect is the mechanical conditioning of the wellbore and any filter cake, reducing permeability and packing any fractures or loss zones with drilling mud and cuttings. However, casing drilling is not applicable to all wells and has not been effective at reducing these problems in all areas and for all well configurations.